Powder mixing or blending is an operation to make two or more powdered substances to form a homogeneous mixture. The operation of mixing finely divided powders consisting of two or more substances is extremely difficult as the particles are subjected to various interparticle forces and such powder can not be set in motion without an external force such as mechanical agitation, ultra sound, electrical forces or similar.
Finely-divided powders are commonly used in inhalation therapy where the size of the particles and the homogeneity of mixtures of substances are of utmost importance. Due to the fact that inhalation therapy is becoming a more and more important therapy not only in the therapy for diseases in the bronchial area but also in therapy against other diseases, the mixing of interacting powders, where a fine, cohesive ingredient may adhere to coarser carrier particles, has become a subject of increased interest during recent years. However, little work has been done regarding the situation where all the ingredients are finely divided, e.g. have a particle size smaller than 10 .mu.m.
For finely-divided powders having a high proportion of particles with particle size smaller than about 10 .mu.m, interparticle adhesive forces, such as van der Waal forces, make the powders cohesive, leading to the formation of irregular aggregates. This formation of aggregates makes the mixing of two or more such cohesive powders much more complicated and difficult than the mixing of powders with a particle size greater than 10 .mu.m. Therefore, if a homogeneous mixture is required, a breakdown of the aggregates must be achieved during the mixing process.
In solid/solid mixing one of the most important requirements is to ensure uniformity of the content, which is particularly relevant for clinical effectiveness when using low dosage cohesive powder mixtures, such as for example those containing 1-2% of the active ingredient. The major problem encountered in powder mixing of finely divided powders is the inability of the commonly used mixers to break down the aggregates formed in the powder. The so-called low-power mixers are not able to breakdown the aggregates formed in the cohesive powders into their primary particles, which means that the aggregates are still present and do not permit the relative movement between particles to occur, something which is necessary if a homogeneous mixture is to be achieved. The critical step of the mixing of low dosage cohesive powder mixtures is the breakdown of the aggregates. Thus, in order to obtain a homogeneous mixture naturally formed aggregates must be repeatedly broken down. To enable the aggregates to breakdown into its primary particles a sufficiently high energy must be applied to the system.